Geochronological results from the Zhela Formation volcanics of the Tethyan Himalaya and their implications for the breakup of eastern Gondwana

The relationship between the Kerguelen mantle plume and the breakup of eastern Gondwana is still debated. The new Zircon SHRIMP U–Pb dating of 139.9 ± 4.6 Ma, as well as previous ages from the Zhela Formation volcanic rocks in the Tethyan Himalaya, show that the studied Zhela Formation volcanic rocks formed during the Late Jurassic-Early Cretaceous, rather than the Middle Jurassic. The calculated volume of the Comei-Bunbury igneous rocks is ~ 114,250 km3, which is compatible with the large igneous provinces and, consequently, the typical mantle plume models. The new date results, along with existing dates, show that the volcanism attributed to the Kerguelen mantle plume in the Tethyan Himalaya ranges from ca.147 Ma to ca.124 Ma, with two peaks at approximately 141 Ma and 133 Ma. This new finding, together with geochemical and palaeomagnetic data obtained from the Comei-Bunbury igneous rocks, indicate that the Kerguelen mantle plume contributed significantly to the breakup of eastern Gondwana and that eastern Gondwana first disintegrated and dispersed at ca.147 Ma, the Indian plate separated completely from the eastern Gondwana before ca.125 Ma.

be attributed to several factors: (1) the small volume (~ 11,000 km 3 ) of the Comei-Bunbury LIP is incompatible with the typical mantle plume models 14 ; and (2) the large distance (~ 1,000 km) between the Bunbury Basalt and the Kerguelen mantle plume 15,16 .
Igneous rocks associated with the Kerguelen mantle plume have been widely disseminated from their initial sites of emplacement due to the movements of the Antarctic, Australian, and Indian Plates 9,[17][18][19] .Tethyan Himalaya was located at the northern margin of India, and thus the Tethyan Himalaya igneous rocks are crucial to understanding the relationship between the Kerguelen mantle plume and the breakup of eastern Gondwana.The close temporal coincidence between the Bunbury Basalt and the Comei igneous rocks (ca.132Ma ago) prompted Zhu et al. 4 to propose that the Comei-Bunbury LIP was probably caused by the Kerguelen mantle plume and that the Kerguelen mantle plume played a vital role in the breakup of eastern Gondwana.However, geochronological

Geological setting and samples
The Himalayan orogen is located at the northern margin of the Indian plate, which is well-known as an active tectonic belt due to the continuous northern movement of the Indian plate.It was divided into four tectonic units including the Tethyan Himalaya, the Greater Himalaya, the Lesser Himalaya, and the sub-Himalaya, from north to south 25 .These tectonic units within the Himalayan orogen are separated by the South Tibetan detachment system, the Main Central thrust, and the Main Boundary thrust from north to south (Fig. 2a).The Tethyan Himalaya, which was located at the northern margin of India, is comprised by Ordovician to Mesozoic sedimentary rocks, partially interlayered with Paleozoic and Mesozoic igneous rocks 25,26 .
The research area is located in the eastern Tethyan Himalaya where Upper Jurassic and Lower Cretaceous strata are well exposed (Fig. 2b).The Upper Jurassic and Lower Cretaceous strata mainly consist of the Zhela, Weimei, Sangxiu, Jiabula, and the Lakang formations.The Weimei Formation, which was deposited parallelly, unconformably underlies the Sangxiu Formation and conformably overlies the Zhela Formation.The contact relationship between the Sangxiu and Jiabula formations is a parallel unconformity.
The Zhela Formation is mainly composed of basalts and dacites interbedded with thin sandstones, slates, and limestones.It is assigned a Middle Jurassic age in the 1: 250,000 scale Longzi Country regional geological survey report (H46C004002, 2005).The Weimei Formation can be subdivided into two members.Member I consists of quartz sandstones, silty slates interbedded with silty metamorphic.Member II includes sericite silty slates interbedded with thin silt sandstones and sand-clastic limestones.The Sangxiu Formation also can be subdivided into two members.Member I consists of silt slates interbedded with sandstones, basalt, and conglomerate.Member II includes sericite silt slate interbedded with sandstones, basalt, and rhyolite.Zircon U-Pb geochronological results reveal that the Sangxiu Formation volcanic rocks formed during ca.136-124Ma 4,22 .The Jiabula Formation mainly consists of quartz sandstone, gray siltstone, bioclastic siliceous rocks, shales, and a small amount of basalt interlayer.The Lakang Formation can be also subdivided into two members.Member I includes basalt interbedded with sedimentary rocks.Member II consists of silt slate, siltstone, and limestone.One fresh sample (TG24) was collected from the Zhela Formation volcanic rocks for zircon SHRIMP U-Pb dating (Fig. 3).Some of the outcrops show apparent amygdaloidal structure (Fig. 3c).The sample is basalt with a characteristic ophitic texture (Fig. 3d,e).The phenocrysts primarily consist of olivine and clinopyroxene.The groundmass mainly includes plagioclase and clinopyroxene.

The ages of the sampling strata
The zircons are subhedral (40-140 μm long, 20-60 μm wide) with weak oscillatory zoning (Fig. 4a and Supplementary Fig. S1).These characteristics, together with the fact that the Th/U ratios (0.11-1.35;Table 1) are obviously greater than the metamorphic zircon ratio (usually < 0.1), suggest that the zircons are of magmatic origin.Zircon SHRIMP U-Pb dating revealed a range of dates (Table 1), implying that these zircons came from various sources.The weighted mean 206 Pb/ 238 U ages of the youngest population are interpreted as the formation time of the volcanic rocks.The sample TG24 yielded 206 Pb/ 238 U ages ranging from 133.7 ± 2.5 Ma to 146.5 ± 2.5 Ma with a weighted average age of 139.9 ± 4.6 Ma (Fig. 4), which indicates that the sampled Zhela Formation volcanic rocks erupted during the Early Cretaceous.Other analyses on zircons yielded older 206 Pb/ 238 U ages ranging from 149.0 ± 2.5 Ma to 3460.0 ± 45.0 Ma, indicating an inherited origin (Supplementary Fig. S1).
Reliable chronological constraints for the sampling strata are vital for regional stratigraphic correlation and division.However, because of the intricate nature of the Tibetan Plateau, numerous layers have been assigned inaccurate ages 19,27,28 .For example, zircon SHRIMP U-Pb dating indicates that the Zhela Formation and Weimei Formation volcanic rocks of the Luozha area in the Tethyan Himalaya erupted during ca.138-135 Ma 19 , not the Middle and Late Jurassic as given by the 1:250,000 scale Luozha regional geological report (H46C004001, 2002).Zircon LA-ICP-MS U-Pb dating indicates that the Risong Formation red beds and volcanic rocks of the Wuma area in the Lhasa terrane formed during the ca.120-106Ma 28 , not the Late Jurassic as given by the 1:250,000 scale Wuma regional geological report (I44C004004, 2006).
Fossils identified in the Zhela Formation include bivalves (Costamussium zandaensis-Quenstedtia xizang ensis assemblages), ammonoids (Dolikephalites-Indocephalites and Dorsetensia-Garantiana assemblages) and belemnites (Holcobelus cf.biainvillei-Hastites and Aractites longissima-Salpigotheuthis assemblages), and are indicative of the Middle Jurassic by the 1:250,000 scale Longzi Country regional geological survey report (H46C004002, 2005).Our new zircon SHRIMP U-Pb dating and recent zircon LA-ICP-MS U-Pb ages (147.1 ± 2.5 Ma) 29 from the same sampling region, however, show that the Zhela Formation volcanic rocks of the Taga area erupted during ca.147-140 Ma, not during the Middle Jurassic.Our updated chronological results are broadly in agreement with the youngest detrital zircons ages of ca.127-138 Ma from the same region reported by Jiao et al. 24 and the ages of ca.135 Ma from the Luozha area reported by Bian et al. 19 .Furthermore, our chronological results are also supported by the bimodal magmatism (144-140 Ma) recently identified in the Taga area by Zhang et al. 30 .

The volume of the Comei-Bunbury LIP
LIPs are usually characterized by the magmatic provinces with areal extents > 100,000 km 2 and igneous volumes > 100,000 km 3 based on Bryan and Ernst 1 .Despite the fact that palaeomagnetic data show that the Comei-Bunbury LIP is positioned in the heart of the Kerguelen mantle plume (see the following section), one essential piece of evidence that must be addressed is if the area and volume of the Comei-Bunbury igneous rocks are comparable to the LIPs.According to Coffin 8 , the area and volume of the Bunbury Basalt are ~ 100,000-1000,000 km 2 and ~ 1,000 km 3 , respectively.In addition, the area and volume of the Comei igneous rocks are ~ 40,000 km 2 based on Zhu et al. 4 and ~ 10,000 km 3 based on Liu et al. 14 , respectively.Although the area matches the LIPs, the small volume (~ 11,000 km 3 ) of the Comei-Bunbury igneous rocks is incompatible with the typical mantle plume models 14 .
Notably, geochronological and geochemical results indicate that the Abor volcanic rocks from Eastern Himalayan Syntaxis in northeastern India 31 , the volcanic rocks from the Naturaliste Plateau, the Wallaby Plateau, and the Mentelle Bassin in southwestern Australia [32][33][34] have geochemical and geochronological similarities to the Comei-Bunbury igneous rocks.The Abor volcanic rocks have an areal extent of ~ 2,500 km 2 with an average thickness of 500 m 35 .The calculated volume of the Abor volcanic rocks is ~ 1,250 km 3 .The Early Cretaceous volcanic rocks from the Naturaliste Plateau (~ 90,000 km 2 ) 34 , the Wallaby Plateau (~ 70,000 km 2 ) 33 , and the Mentelle Bassin (~ 44,000 km 2 ) 32 have an areal extent of ~ 204,000 km 2 .Assuming an average thickness of 500-1000 m, the minimum volume of those volcanic rocks is 102,000 km 3 .Totally, the calculated volume of the Comei-Bunbury igneous rocks is ~ 114,250 km 3 .Therefore, the volume of Comei-Bunbury igneous rocks is compatible with the LIPs, and thus the typical mantle plume models.

The palaeolatitudes of the Comei-Bunbury LIP
Because of the movements of the Indian, Australian, and Antarctic plates, igneous rocks associated with the Kerguelen mantle plume have been widely scattered from their initial positions of emplacement.Understanding the spatial interaction of these igneous rocks with the Kerguelen plume mantle requires determining their initial positions of emplacement.Palaeomagnetism is the only way to quantify the palaeolatitude of the plate 36 and is thus essential to constrain the erupted position of these igneous rocks.Table 2 lists the reliable latest Jurassic to Early Cretaceous palaeomagnetic data from the Comei-Bunbury igneous rocks.Notably, based on the 1:250,000 Longzi regional geological survey report (H46C004002, 2004), Yang et al. 21assigned an Early Cretaceous date (134-131 Ma) to the Lakang Formation lava flows in the Cona area in the eastern Tethyan Himalaya.According to recent zircon LA-ICP-MS and SHRIMP U-Pb geochronological data, the Lakang Formation lava flows erupted at ca. 147-141 Ma 10,19,23 .The Abor volcanic rocks of the northeastern Indian craton were given an Early Permian date by Ali et al. 37 .New zircon LA-ICP-MS U-Pb analyses, however, have shown that the Abor volcanic rocks erupted at ca.133-131 Ma 31 .Additionally, based on K/Ar dating performed by McDougall and Wellman 38 , Schmidt 39 gave a Late Cretaceous age to the Bunbury Basalt of southern Australia.According to new 40 Ar/ 39 Ar analytical results, the Bunbury Basalt erupted at ca.137-130 Ma 12 .This study made use of these fresh geochronological findings.
The palaeopole (TG 29 ) obtained from the Zhela Formation volcanic rocks in the Taga area of the eastern Tethyan Himalaya is well constrained by our new zircon U-Pb geochronology and yielded a palaeolatitude of 44.1° ± 11.8°S at ca.147-140 Ma for the reference point (28.4°N,91.8°E) (Table 2).Furthermore, five palaeopoles obtained from the Cona (LK 21 and MC 40 ), Zhuode (ZD 19 ), and Langkazi (SX 22 and ZY 41 ) areas of the eastern Tethyan Himalaya yielded palaeolatitudes of 52.5 ± 5.7°S at ca.147-141 Ma, 39.6 ± 7.6°S at ca.137-125 Ma,   2).The palaeolatitude of the Taga area is identical to the palaeolatitudes obtained from the Cona, Zhuode, and Langkazi areas within 95% confidence limits, indicating that the eastern Tethyan Himalaya was located at ~ 39.6-53.5°Sduring ca.147-124Ma.Similarly, the palaeopole (Ab 37 ) obtained from the Abor volcanic rocks in the northeastern Indian craton yielded a palaeolatitude of 55.5 ± 8.6°S at ca.133-131 Ma for the reference point (28.3°N,95.1°E) (Table 2).The palaeopole (BB 39 ) obtained from the Bunbury Basalt in southwestern Australia yielded a palaeolatitude of 52.0 ± 4.0°S at ca.137-130 for the reference point (33.8°S,115.6°E) (Table 2).These palaeomagnetic results indicate that the Comei-Bunbury igneous rocks, which are presently located at 28.1-28.9°Nfor the eastern Tethyan Himalaya, at 28.3°N for the northeastern Indian craton, and at 33.3-34.3°Sfor southwestern Australia, originally erupted at ~ 39.6-55.5°S.According to the hybrid reference frames described by Torsvik et al. 42 , the eruption center of the reconstructed Kerguelen mantle plume LIPs was located at ~ 41.6-52.3°S.The palaeolatitudes of the reconstructed Kerguelen mantle plume LIPs are identical to the palaeolatitudes of the Comei-Bunbury LIP (~ 39.6-55.5°S),indicating that the Comei-Bunbury igneous rocks came from the Kerguelen mantle plume.

Implications for the breakup of eastern Gondwana
Because of the small volume of the Comei-Bunbury igneous rocks 13,14 , the large distance between the Bunbury Basalt and the Kerguelen mantle plume 15,16 , the relationship between the Comei-Bunbury igneous rocks and the Kerguelen mantle plume is still debated.Geochronological and geochemical results from the mafic rocks in the Taga area show that these mafic rocks erupted during ca.144-140 Ma and that bimodal magmatism was identified as a response to early Kerguelen plume mantle activity 30 .Consider the similarity in sample area and age between the mafic rocks reported by Zhang et al. 30 and the basalt investigated in this study, implying that they originated from the same source, the Kerguelen mantle plume.This is consistent with the geochronological and geochemical results from the Zhela formation volcanic rocks in the Zhuode area of the eastern Tethyan Himalaya 6 .The Sangxiu, Lakang, Zhela, and Weimei formations volcanic rocks in the eastern Tethyan Himalaya showed no obvious Eu anomalies and shared geochemical characteristics that high contents of TiO 2 , highly fractionated in light rare earth elements and heavy rare earth elements, and indicated similarities to the alkali basalts originated from the Kerguelen mantle plume 6,9,10 .This, together with the recalculated volume of the Comei-Bunbury igneous rocks and the spatial relationships between the Comei-Bunbury igneous rocks and the Kerguelen mantle plume as we mentioned above, indicate that the Comei-Bunbury igneous rocks originated from the Kerguelen mantle plume.
According to zircon SHRIMP U-Pb dating and geochemical study from the Lakang Formation volcanic rocks in the eastern Tethyan Himalaya, the Kerguelen mantle plume began in the Late Jurassic (ca.147Ma), and it played an essential part in the breakup of eastern Gondwana 10 .Furthermore, gabbro-diabase and gabbro samples from the Rongbu area in the eastern Tethyan Himalaya share geochemical characteristics with the volcanic rocks of the Lakang and Sangxiu formations, which both formed on the continental margin in a significantly extensional rift zone 43 .These findings suggest that the Kerguelen mantle plume's activity began at ca. 147 Ma and that eastern Gondwana first disintegrated and dispersed at that time.However, no volcanic rocks older than 137 Ma have been discovered in southwestern Australia.This might be explained by the fact that the eastern Tethyan Himalaya was situated in the center of the plume head (Fig. 6a 11,30,57,58 ).
With the breakup of the eastern Gondwana and the growth of mantle plumes, the Indian plate started moving northward after ca.140 Ma 59 , and the oldest oceanic crust (ca.140Ma) off western Australia grew younger to the west 60 .Furthermore, marine magnetic anomalies for the west Australian margin suggested that the midocean ridge that separated the Indian plate from the Australian-Antarctic plate began at ~ 136 Ma.According to 40 Ar/ 39 Ar geochronological and whole-rock geochemical studies, the Bunbury Basalt erupted in three separate phases (ca.137 Ma, ca.133 Ma, and ca.130 Ma) and shared geochemical similarities with the Kerguelen mantle plume-products 12 .Olierook et al. 33 suggested that the Bunbury Basalt erupted in the last two phases originated from the same flow.Therefore, the primary eruption period of the Bunbury Basalt is generally consistent with that of the Comei igneous rocks, both of which may have peaked during 141-137 Ma and 133-130 Ma (Fig. 6b,c).
The eastern Tethyan Himalaya advanced to the edge of the mantle plume when the Indian plate moved northward, generating the ca.125 Ma OIB magmatic rocks 11 .The bimodal magnetic rocks (118-115 Ma) and N-MORB-like mafic rocks (ca.120Ma) discovered in the eastern Tethyan Himalaya are the result of an extensional environment and are not the products of Kerguelen mantle plumes 45,61 .These findings, together with the fact that the Australian-Antarctic plate maintained a relatively stable palaeolatitude during 140-120 Ma 59 , suggest that the Indian plate separated completely from the eastern Gondwana before ca.125 Ma (Fig. 6d).

Conclusions
We have presented new zircon SHRIMP U-Pb geochronological results from the Zhela Formation volcanic rocks in the Tethyan Himalaya.Our new results, together with previous geochronological, geochemical, and palaeomagnetic data from the Comei-Bunbury LIP, led us to draw the following conclusions: (1) the studied Zhela Formation volcanic rocks formed during ca.147-140Ma, rather than the Middle Jurassic as assigned by the 1:250,000 scale Longzi regional geological survey report; (2) the calculated volume of the Comei-Bunbury  S1 for data compilation and references.(b) Simulation of kernel density estimate variance using 100 trials where dates are randomly selected for each of the ages in Supplementary Table S1 using their ages and uncertainties 56 .Two potential peaks can be identified at ca.

Methods
One fresh block sample was collected from the Zhela Formation volcanic rocks near Taga village located at ~ 45 km west of Longzi town (28°22′11.4″N,91°47′12.6″E) in the eastern Tethyan Himalaya.Zircons for SHRIMP U-Pb dating were extracted by magnetic cleaning and heavy mineral separation from crushed samples and finally selected by hand-picking under the binoculars.The selected zircons were mounted onto an epoxy resin disc together with some standard zircon grains and then ground down and polished to expose their interiors.Transmission, reflected and cathodoluminescence images photographed by optical microscopy were used to check their internal structures for subsequent SHRIMP U-Pb dating.
Zircon SHRIMP U-Pb dating was conducted at SHRIMP IIe at the Beijing SHRIMP Center, Institute of Geology, Chinese Academy of Geological Sciences, Beijing, China.Software packages Squid 62 and Isoplot 63 were used to process the data.During testing, the mass resolution was 5,000 (1% definition), the spot sizes were 25-30 μm, and the ion flow intensity of O 2 -was 4 nA.The U and Th contents of the unidentified zircon particles were calibrated using a standard zircon sample M257.To ensure the precision and reliability of the experimental data, the standard TEMORA zircon grain calibration was carried out after every fourth analysis.The weighted mean ages are provided at the 95% confidence interval, while the uncertainties for individual analysis are quoted at the 1-sigma level.

Figure 1 .
Figure 1.Temporal and spatial distribution of products related to the Kerguelen mantle plume (after Zhu et al. 9 ).

Figure 4 .
Figure 4. (a) Cathodoluminescence images of representative zircon grains and corresponding younger 206 Pb/ 238 U ages of the analyzed spots.(b) U-Pb concordia diagram of zircon grains.(c) Bar plot shows the weighted mean 206 Pb/ 238 U ages.

Figure 5 .
Figure 5. (a) Zircon U-Pb ages for samples of the Comei LIP in the Tethyan Himalaya.See Supplementary TableS1for data compilation and references.(b) Simulation of kernel density estimate variance using 100 trials where dates are randomly selected for each of the ages in Supplementary TableS1using their ages and uncertainties56 .Two potential peaks can be identified at ca. 141 Ma and ca.133 Ma in 97 trials in the kernel density estimate plot.Abbreviations: KDE, kernel density estimate.
Figure 5. (a) Zircon U-Pb ages for samples of the Comei LIP in the Tethyan Himalaya.See Supplementary TableS1for data compilation and references.(b) Simulation of kernel density estimate variance using 100 trials where dates are randomly selected for each of the ages in Supplementary TableS1using their ages and uncertainties56 .Two potential peaks can be identified at ca. 141 Ma and ca.133 Ma in 97 trials in the kernel density estimate plot.Abbreviations: KDE, kernel density estimate.

Table 1 .
Zircon SHRIMP U-Pb ages for sample TG24 in the Taga area of the eastern Tethyan Himalaya.Pb c indicates common Pb; Pb* indicates radiogenic Pb; Errors are 1σ; Spot ID with * are discarded because of its higher common Pb.